US 2001/0002782 corresponding to JP-A-2001-169407 discloses a power converter used for control of a motor which is mounted on an electric vehicle to drive the vehicle. The power converter includes an inverter circuit to convert direct-current (DC) power to alternating-current (AC) power.
Generally, an inverter circuit used for control of a motor is connected to a DC power source and includes upper and lower switching devices that are connected in series to form a so-called leg of the inverter circuit. The inverter circuit supplies power to an inductive load from a connection point between the upper and lower switching devices. For example, an inverter circuit for a three-phase motor has three legs that are connected in a bridge configuration. PWM signals are applied to switching devices of the inverter circuit, and the switching devices are switched ON and OFF in accordance with the PWM signals. Thus, the inverter circuit generates AC power from the DC power source.
In an inverter circuit that drives an inductive load, a diode is connected in antiparallel with each switching device. When one of upper and lower switching devices is turned OFF, an electric current circulates through the inductive load and the diode of the other of the upper and lower switching devices. Typically, a dead-time, where both the upper and lower switching devices are kept OFF, is provided to prevent the upper and lower switching from being ON at the same time, thereby avoiding a short-circuit in a power source.
In practice, a power converter including an inverter circuit is required to achieve a stable and efficient operation by overcoming two challenges that have a trade-off relationship. The first challenge is to protect a switching device from a surge voltage. The second challenge is to reduce loss. Specifically, in a switching device such as an IGBT or a power MOS transistor used in an inverter circuit, although a switching loss (i.e., heat generation) decreases with an increase in a switching speed, a surge voltage increases with the increase in the switching speed. That is, as the switching speed at which the switching device is turned ON and OFF in accordance with the PWM signal becomes faster, the surge voltage becomes larger. When the surge voltage exceeds an allowable breakdown voltage of the switching device, the switching device is broken. Therefore, to ensure a stable operation of the switching device, the switching speed is so slow that the surge voltage can be smaller than the breakdown voltage. One approach to reduce the switching speed is to connect an input resistor (i.e., gate resistor) in series with a gate of the switching device so that rising and falling edges of a waveform of a gate signal can be smoothed. However, this approach increases a switching loss, and therefore it is difficult to achieve an efficient operation of the switching device. Further, since the increase in the switching loss causes an increase in heat generated in the switching device, there is a possibility that a temperature of the switching device exceeds an allowable temperature of the switching device. For these reasons, if it is impossible to set the switching speed to satisfy both the allowable breakdown voltage and the allowable temperature, there is a need to increase the breakdown voltage and/or the size of the switching device. As a result, the size and cost of the switching device are increased.
In a technique disclosed in US 2001/0002782, the switching speed is changed based on the temperature of the switching device to protect the switching device from the surge voltage and to reduce the switching loss. However, according to the technique, the dead-time is determined based on when the switching speed is slow. Therefore, when the switching speed is fast, the dead-time is too long. The excessive dead-time degrades a voltage utilization factor of a power converter and also causes a ripple in output voltage. Therefore, for example, when the technique is applied to a power converter for a motor, an available RPM-torque range of the motor may be narrowed, and also quietness may be degraded due to a torque ripple.